Devices for measuring speed and/or distance of travel on vehicles, such as bicycles, have been available for many years. Most such devices are purely mechanical in nature and utilize front wheel mounted mechanical units and a cable to actuate conventional mechanical speedometer/odometer counting devices. Such systems are generally inefficient and provide undesirable weight or pressure on the wheel so as to create unnecessary drag thereon. Further, such mechanical devices are subject to malfunctions and require a reasonably high degree of maintenance and repair.
Efforts to improve over such conventional mechanical devices have been tried by those in the art. For example, the device described in U.S. Pat. No. 3,898,563, issued on Aug. 5, 1975 to D. E. Erisman, utilizes one or more magnets symmetrically attached to the drive sprocket of the bicycle and a coil attached to the bicycle frame. Alternatively, the Erisman device utilizes a plurality of magnets symmetrically attached to the spokes of the front or rear wheels, together with a sensor housing mounted on the wheel fork.
U.S. Pat. No. 4,007,419, issued on Feb. 8, 1977 to R. Jasmine, describes the use of coil mounted on a wheel fork and a plurality of magnets mounted on the wheel sprocket which generate a voltage which is processed to provide an indication of the speed or distance the vehicle has travelled.
In each of the aforesaid issued patents a magnet is placed on the spokes and thereby extra, and undesirable, weight is added to the wheel which would be detrimental to the balance of the wheel and would increase the moment of inertia thereof.
U.S. Pat. No. 4,156,190, issued May 22, 1970 to B. C. Chittendent et al. discloses a system wherein each spoke or valve stem interrupts a light beam from a light-emitting diode which is in alignment with a phototransistor, the latter device being mounted on the wheel frame. The number of light beam interruptions is utilized to compute the speed and/or distance of travel of the vehicle. Such a device introduces complicated alignment problems between the light-emitting diode and the phototransistor arrangement and such system is highly susceptible to false signals. Moreover, the device has a relatively large number of components, such components not only increasing the cost thereof but also requiring a relatively large amount of power necessitating the use of a relatively heavy battery power source.
It is desirable, therefore, to provide a bicycle speedometer/odometer system in which no weight is added to, and no pressure is applied to, the wheel itself and which is not affected by dirt or mud which may accumulate on the sensor or by reflections from the wheel or spokes thereof and provide spurious readings. Such a device should provide high measurement accuracy, be relatively simple to install, be less susceptible to spurious signals, and require no complicated alignment procedures. Further, such a device should be relatively easy to calibrate and provide effective operation at a relatively low cost.